S-waves are a fundamental type of seismic energy that travels through the Earth’s interior. The ‘S’ stands for “Secondary Wave” or “Shear Wave,” both describing key aspects of their behavior. They are categorized as body waves, meaning they propagate through the bulk of a material, unlike surface waves that are confined to the outer layers. S-waves are generated primarily by earthquakes and are recorded by seismographs worldwide.
Defining S-Waves: The Mechanics of Shear Motion
The physical mechanism of an S-wave involves a distinct shearing motion as it travels through a solid medium. S-waves are classified as transverse waves, meaning the material particles move perpendicular to the direction the wave is advancing. For example, if the wave travels horizontally, the material moves vertically or laterally. This displacement is the shear motion that defines the S-wave.
S-wave energy propagation relies on the material’s ability to resist and recover from sideways deformation, a property known as shear strength or rigidity. This motion differs fundamentally from P-waves, which involve compression and expansion in the direction the wave travels. Because S-waves depend on shear strength, they cannot propagate through fluids, which lack the necessary rigidity to transmit the shearing force. The speed of S-waves is about 60% that of P-waves in the same material, which is why they arrive second at a seismic station.
How S-Waves Reveal Earth’s Internal Structure
The inability of S-waves to travel through non-rigid media allows scientists to map the Earth’s deep structure. When an S-wave encounters a liquid, it is stopped or converted into other wave types. This principle proved the existence of the Earth’s liquid outer core, a layer composed primarily of molten iron and nickel.
Seismologists observed that S-waves generated by earthquakes never arrive at seismic stations opposite the epicenter. This large area, known as the S-wave shadow zone, results from the waves being blocked by the liquid outer core. Mapping the boundaries of this shadow zone allowed scientists to determine the size and depth of the liquid layer surrounding the solid inner core. S-wave data confirms the layered structure and physical state of materials deep within the planet.
The Role of S-Waves in Earthquake Damage
S-waves are responsible for the most severe damage experienced during an earthquake due to their specific motion and arrival time. They are called “secondary” because they travel slower than the initial P-waves, arriving second at any given point. This delay provides a small window of time for early warning systems.
The transverse, side-to-side, and up-and-down shaking motion of S-waves is far more destructive than the gentler push-and-pull of P-waves. Buildings are designed to withstand vertical loads but are vulnerable to the lateral, shearing forces delivered by S-waves. S-waves exhibit a significantly larger amplitude of ground motion compared to P-waves at the same distance from the epicenter. This greater displacement causes walls to shear, foundations to crack, and structures to collapse.
The time difference between the arrival of the faster P-wave and the slower S-wave is directly proportional to the distance from the seismic station to the earthquake’s epicenter. Seismologists use this S-P interval, recorded on a seismogram, to calculate the distance to the earthquake source. Data from at least three seismic stations are then used in a process called triangulation to pinpoint the location of the epicenter.